You can see one of the wicked worms in my thumbnail photo. Every June I promise myself I'll get ahead of them the following spring, and always forget to do so until my rose leaves are turning a mottled brown. By then, it's far too late.
There are actually three types of rose slugs. I'm guessing that the ones I've photographed are probably the bristly variety, Cladius difformis. Although I haven't noticed any five o'clock shadow on them, I almost always find them on the undersides of the leaves, which is where bristly rose slugs feed. The smooth European type--Endelomyia aethiops, on the other hand, prefer to flaunt themselves on the tops. There is also a less common curled slug, called Allantus cinctus, that haunts the "underworld" like the unshaven types! All of the little green menaces are actually the larvae of sawflies, which look something like small wasps with see-through wings and no waists.
Theoretically, sawfly larvae should be easy enough to kill. They are supposed to be vulnerable to a wide variety of predators, including birds, beetles, and wasps, and most of the organic insecticides, including horticultural oil, insecticidal soap, neem, spinosad, and pyola.
It must be difficult for predators to see slugs that stay hidden, however. The ungrateful birds that I feed all winter don't seem to pay much attention to them anyway! The bristly and European slugs also drop off the plants once they've gorged themselves and go underground to pupate. So dormant sprays--unless applied at exactly the right time--aren't going to kill any of the slugs except the curled ones, which pupate in the roses' canes.
That right time would probably be about early May when the sawflies emerge to lay their eggs in little slits in the rose leaves. If I could just remember to douse the bushes with a horticultural oil solution at that time, I might be able to smother the slugs before they hatch. Those organic insecticides mentioned earlier should work to kill the worms after they hatch, or you can just blast them off the plants with a strong stream of water. Some heirloom rose bushes can be at least six feet across, however, and thoroughly treating the undersides of all their leaves is next to impossible.
Although I once had the idea that the slugs preferred heirloom over more modern roses, I've decided that it's is more a matter of how long any rose bushes have remained in one place. The heirlooms have generally had more years to build up slug populations around them. My most recently planted rose bed seems the least affected, though it will probably get hit harder over time.
I've been debating whether there's any way to actually kill the larvae while they are in the soil. Some people say that disturbing the earth around the bushes during the winter or early spring will do it. I've also seen spinosad mentioned as a possible soil drench. If anybody knows whether that really works or not, please inform me. I would rather not spray spinosad directly on the rose leaves, as I've heard it can kill bees too.
I sometimes get similar slugs on plants in the mallow family such as hardy hibiscus and hollyhock. They are probably the larvae of the mallow or hibiscus sawfly instead. So keep an eye on the undersides of those leaves too.
In the meantime, I'm going to use one of those memo programs to send myself an e-mail message next May. Something to the effect of: "DROP WHATEVER YOU ARE DOING AND OIL THOSE ROSES!"
Photos: The rose slug photos are mine. The sawfly photo is by R. Berg, courtesy of BugGuide.